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What is a live attenuated (wild type) vaccine?

Live attenuated vaccines contain a living, although significantly weakened, version of a pathogen (a bacterium, virus or other microorganism that can make you sick). Measles, mumps and chicken pox vaccines are made with live viruses. The only bacterial vaccine made with live pathogens in the United States is the typhoid vaccine.

The benefit of a live vaccine is that a single dose often provides lifelong immunity. The downside is that because viruses and other pathogens naturally mutate, or change, the virus within the vaccine could also change, possibly creating a stronger version of itself that the immune system would have difficulty combating. This was an issue with the early oral polio vaccines but is generally not a problem with current live vaccines, which are much safer than the virus they protect against.

Only people with a suppressed immune system (such as those who have HIV/AIDS, are taking immunosuppressant drugs or are being treated for cancer) should be concerned about receiving live vaccines because they could, conceivably, become infected with the virus. Live vaccines also usually require refrigeration.

Live attenuated vaccines—including those for measles, mumps, chickenpox, shingles and rubella—contain a live microbe that is weakened in such a way that it can no longer cause disease. Live attenuated vaccines elicit a strong immune response, involving both memory B cells and memory T cells, and can confer lifelong immunity after as few as one or two doses.

A live attenuated vaccine is composed of a living wild type virus or bacteria that has been changed in the laboratory so that it can grow and replicate but not cause illness. In this way, a person's immune system can recognize the virus or bacteria and form immunity to it without the person contracting the actual disease.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.